1. Field of the Invention
The present invention relates in general to magnetically coupled pumps of the type having a rotary impeller which is rigidly attached to a cylindrical permanent magnet that is driven by a torque from a surrounding annular permanent magnet, the two magnets being separated from one another by a wall of the pump housing. In particular, the present invention relates to a rotary impeller which is more easily attached to the driven magnet than impellers known in the prior art.
2. Description of the Prior Art
Magnetically coupled pumps are well known in the prior art as evidenced by U.S. Pat. Nos. 3,205,827; 3,465,681; 3,545,892; 3,802,804; 3,932,068 and 3,938,914. These pumps are of a type having a rotary impeller having an integral hub portion upon which is mounted a hollow cylindrical driven magnet. The driven magnet is mounted by the insertion of the hub portion of the impeller into the bore of the magnet. The driven magnet, in turn, is rotated by a torque supplied by an annular driving magnet which is separated from the driven magnet by a wall of the pump housing. The rotation of the driven magnet thus drives the impeller.
One long-felt problem associated with such prior art pumps has been to provide a satisfactory means for fastening the impeller hub to the driven magnet. If the driven magnet is not adequately fastened to the hub, slippage will result in that the driven magnet will rotate while the hub and the impeller blades remain stationary. In order to alleviate this problem, it has been common to form the bore of the hollow cylindrical driven magnet such that it is only slightly larger than the impeller hub. In this manner, friction between the driven magnet and the hub will eliminate any slippage. However, sintered ceramic magnets have been preferred for use as the driven magnet because of their ability to retain flux and these sintered magnets are easily cracked and chipped when being force fit onto the impeller hub. Further, the precision grinding of the bore of these magnets is an expensive manufacturing operation which increases the cost of the pump.
Another problem associated with prior art magnetically coupled pumps, especially those with molded plastic impellers, is wear caused by the rotation of moving parts such as the impeller over stationary surfaces.